The present invention relates to apparatus for plasticizing and injecting mouldable synthetic material.
Injection casting machines and injection presses for the manufacture of moulded parts of synthetic material generally comprise a hollow cylinder provided with a worm shaft, which is rotatably mounted in the bore of the cylinder and which makes possible the conveying the plasticizing of mouldable synthetic material. In many cases, the worm shaft is also axially displaceable and then serves simultaneously as a piston to press the synthetic material through a nozzle in the moulding tool.
Earlier, the time expended for the manufacture of a moulded part was largely determined by the time required for the cooling down of the moulded part. In more recent times, moulding tools have been improved in such a manner, that the cooling-down time became substantially shorter than was the case previously. This is the case particularly in the manufacture of thin-walled parts, such as for example containers, pots, lids and wrapping materials.
During the cooling-down time, different operating steps, particularly the plasticizing of the mouldable synthetic material, have been carried out. The shortening of the cooling down time is now such that the manufacturing time for thin-walled moulded parts is determined mainly by the expenditure of time required for the plasticizing the mouldable material.
The plasticizing performance of a press can be increased if the circumference of the worm shaft is enlarged. When the worm shaft is displaced in an axial direction towards the nozzle to press the mouldable material through the nozzle, the driving force required to axially displace the shaft is equal to the product of the cross-sectional area of the worm shaft and the pressure exerted on the mouldable material by the worm shaft. If the same pressure is to be attained with worm shafts of different diameters, the driving force in the case of the use of a worm shaft of larger diameter must thus be increased proportionally to the greater cross-sectional area of such an enlarged worm shaft.
This however poses a problem since the manufacturer of the injection casting machines and injection presses normally manufactures about three to four standard sizes of each type of machine, the drive motors of which are matched to the sizes of the worm shaft diameter. If the diameter of the worm shaft is now increased, then the performance of the motor serving for the generation of the pressure must be correspondingly increased. The employment of a stronger motor for the generation of the greater pressure would however make the machine appreciably more expensive and also render it considerably more bulky.
There is of course the possibility of employing a hydraulic pressure converter in place of a stronger motor. This solution is also however relatively expensive. Moreover, greater axial forces, which represent an increased loading of the machine elements, arise in the employment of a stronger motor as well as also in the employment of a pressure converter.
An object of the present invention is to provide an apparatus, which makes it possible to plasticize a greater quantity of mouldable synthetic material per unit time without the attainable pressure decreasing for that reason and without the driving equipment serving for the axial displacement of the worm shaft having to be changed significantly. Furthermore, the increase in plasticizing performance should also not materially increase the bulk and the cost of the machine.